The keen-eyed among you will notice that there is a new photograph of me at the top of this column. I have asked them not to use the one that makes me look like a sex offender but I have noticed that pretty much all pictures of me look like that these days. (One is not pleasantly reminded of Orwell's dictum that at 40 everyone has the face they deserve. Do I really deserve to look like that? It's pretty hard to commit any kind of offence with three kids running around the place.)

The most striking instance of this was when I renewed my British Library pass the other day. The mug shots for these are taken at five-year intervals, for that is how long the passes last. The difference between the last snap taken and the new one is so striking that when it slipped out of the machine, I fell out of the chair with a spasm of loathing.

"Dear God," I said. What was worse was that the member of staff I was dealing with also said something along those lines. It was like a cunning reversal of Dorian Gray's portrait. (I have, in my attic, a secret painting that preserves me as an eternally youthful twentysomething. Spooky, huh?)

So, I have been asking myself, what the hell has happened. And all I can think of is the accumulated years of childcare, which, like sentences for multiple convictions, run concurrently. Let me illustrate. When I worked in a proper office, I remember a few of us being given a peptalk by a manager. "How long have you worked here?" he asked. "Two years," I said miserably. Round the table came the other answers to the same question. The manager added up the total. "You see? Between you, you have over 20 years' experience in marketing." We looked at him hollowly. It doesn't work like that, our mutely reproachful gazes told him.

Well, in childcare it does work like that. Between 1997 and now, I have not aged five years; I have aged almost 12 years. In other words, although the oldest child is only seven, I have in fact - if you do the simple maths of adding up their ages - done nearly 14 years of assorted babysitting, nappy-changing, bathing, fretting and whatnot. And in all that time I have had, at a reasonable estimate, about five nights of uninterrupted sleep. And such sleep comes only after stunning myself with alcohol, like a character in a Kingsley Amis novel. No wonder I look like crap.

Ah, you say, but you are a Slack Dad. You have been making life as easy as possible for yourself. You do not pull your weight. To which I can only reply that that is the theory, but I must be doing something wrong - or maybe this business of being slack is, paradoxically, taking up too much time and effort. It is hard, now I come to think of it. The point is, even the slackest dad has to do something if he is to remain on site, as opposed to divorced and living round the corner in a shabby flat.

But those tiresome duties... It is not like work, where doing it is rewarded with money and the esteem of your peers. At home, where un-slack mum occasionally gets fed up with doing almost everything, the system of reward is not so much carrot-and-stick as stick-and-absence-of-stick. After a while, this seems like a pretty fair deal, or at least inevitable in a fatalistic way. So Slack Dad gets accustomed to occasional clouts with a piece of two-by-four when he drifts off into that serene, private world in his head where nothing around the house has to get done. The search is for that blissful condition of stasis that is so essential to the condition of true slackness. I like to think there is something Zen about it.

But children are not very Zen. They do not achieve stasis. They grow. They also run around, bounce up and down, demand entertainment. One tries to steer them gently in the direction of the television, which is awfully educational, but they don't always take the bait. And even the dimmest, most selfish Slack Dad knows that this is how children are meant to be. Meanwhile, I look with a surly and suspicious eye at the dads who have even more energy than their children, those who take their offspring to the theatre all the time. Or at least when they are not proactively stringing up interesting decorations made from washing-up liquid bottles and sticky-backed plastic.

Take it easy, I feel like telling them. In a few years' time you're going to look really awful. Only, for some reason, they never do.